The craze for bumper stickers seems to have passed in UK, but it was "all the rage" a few years ago, with all the usual ones: "My other car is a Porsche" (which was really funny when I saw this on a Porsche!); "If you can read this you're too ******* close!"; a pair of stickers either side of the bumper labelled "Overtaking Side" and "Suicide"; "Don't laugh at this sticker - it may be your wife inside!" and similar.
All of the really good ones were topical, and would not be funny now, I guess, even if my grey cells were up to such a memory feat.